1. Field of the Invention
The present invention relates to a circuit and a method for detecting a low capacitance and a fingerprint sensor using the circuit.
2. Description of the Related Art
Pressure sensitive capacitive sensors have been developed as a fingerprint sensor that is considered as the most promising technique in biometrics. The pressure sensitive capacitive sensor includes one film bearing column lines spaced at predetermined intervals on the surface thereof and the other film bearing row lines spaced at predetermined intervals on the surface thereof. The two films are arranged to face each other with an insulator interposed therebetween to allow a predetermined separation therebetween. The films of the pressure sensitive capacitive sensor are deformed in response to the roughness of the fingerprint when a finger is placed onto the sensor. The separation between the column line and the row line changes depending on location, and the shape of the fingerprint is detected as a capacitance at the intersection of the lines. One of the techniques available to detect a capacitance less than several hundreds of fF (femto farads) is a detector circuit that converts a capacitance into an electrical signal using a switched capacitor circuit. In the detector circuit, a common switched capacitor circuit is connected to a sensor capacitance element, driven by a first drive signal, for detecting a target capacitance, and a reference capacitance element, driven by a second drive signal and serving as a detector circuit reference capacitance. First and second sample-and-hold units, which alternately functions, sample the respective output signals, and determine a difference between the sample results to result in a detected signal.
The common capacitor switched circuit in the detector reliably detects a signal proportional to a capacitance value Cs as a target and inversely proportional to a feedback capacitance Cf. Although a charge Qd are stored in a parasitic capacitance between the gate electrode of a reset switch (feedback control switch) and another electrode in the switched capacitor circuit, the effect of leakage (field through) of the charge to the other electrode is cancelled. As disclosed in Japanese Unexamined Patent Application Publication No. 8-145717, it is also expected that low-frequency noise contained in an offset component at a reference voltage in the switched capacitor circuit and an input signal is removed to some degree by determining a difference between the two sample results.
High sensitivity is required of the capacitance detector circuit such as the fingerprint sensor because of a small capacitance change, while the detector must be robust against noise coming in from the human body (including high-frequency components) and noise derived from electrical circuits.
The detector must also be free from the effect of crosstalk noise from adjacent lines among column lines and row lines that detect a change in capacitance.
One contemplated capacitance detector circuit detects a charge voltage corresponding to a charge stored in a capacitance at an intersection of a column line and a row line when the column line rises in a voltage level, and a discharge voltage corresponding to a charge discharged from the capacitance at the intersection when the column line falls in voltage. The detector circuit thus detects a change in capacitance using the charge voltage and the discharge voltage.
More specifically, the capacitance detector circuit determines a difference between the charge voltage and the discharge voltage, and regards the difference voltage as a voltage corresponding to a change in capacitance. The detector circuit thus removes a voltage offset arising from the effect of field through at the same electrodes in an amplifier circuit, and an offset component taking place in other circuits, thereby eliminating noise sufficiently low in frequency in comparison with a sampling frequency.
When a change in capacitance of each sensor element of a capacitive sensor is detected, typical detector circuits including a capacitance detector circuit drives a single column line only, thereby detecting a change in a capacitance value Cs at each of intersections (sensor elements) of a plurality of row lines, serving detecting lines, and the column line.
As previously discussed, a change in capacitance per sensor element (single intersection) is as small as several hundreds of fF.
Known capacitance detector circuits are still subject to the effect of noise occurring in the capacitive sensor even if the offset component taking place in circuits containing an amplifier is removed.
In other words, the capacitance detector circuit is unable to precisely detect a capacitance change because of disturbing noise when power supply noise and conductive noise transferred to a capacitive sensor via the human body are superimposed on a signal of the column line and the row line.
Widely available inverter type fluorescent lamps generating high-frequency through semiconductors become a noise source having a fundamental frequency of several tens of kHz.
When the difference between the charge voltage and the discharge voltage is detected in the capacitance detector circuit, the sampling frequency of the capacitance change becomes close to the fundamental frequency of the noise source.
When the capacitance detector circuit determines the difference between the charge voltage and the discharge voltage, a beat due to the difference remains. Such a beam is generated when two waves having slightly different frequencies are superimposed. Thus, it is difficult to completely remove disturbing noise components.
When a user uses a fingerprint sensor, an apparatus having a noise source of a frequency component close to the sample frequency of the capacitance detector circuit may be located close to the body of the user. For example, an inverter type fluorescent lamp may be used, or an apparatus having an inverter circuit used to backlight a liquid-crystal display may be connected to the sensor. The disturbance noise due to the beat cannot be fully eliminated. The signal to noise ratio S/N of s signal for detecting a capacitance change is degraded, and the fingerprint of a user cannot be precisely read.